MAGNETIC EFFECT OF CURRENT
Question 18:
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Why does a compass needle get deflected when brought near a
bar magnet?
A compass needle is
a small bar magnet. When it is brought near a bar magnet, its magnetic field
lines interact with that of the bar magnet. Hence, a compass needle shows a
deflection when brought near the bar magnet.
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Draw magnetic field lines around a bar magnet.
Magnetic field lines
of a bar magnet emerge from the north pole and terminate at the south pole.
Inside the magnet, the field lines emerge from the south pole and terminate
at the north pole, as shown in the given figure.
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List the properties of magnetic lines of force.
The properties of
magnetic lines of force are as follows.
(a) Magnetic field
lines emerge from the north pole.
(b) They merge at the
south pole.
(c) The direction of
field lines inside the magnet is from the south pole to the north pole.
(d) Magnetic lines do
not intersect with each other.
Why don’t two magnetic
lines of force intersect each other?
If two field lines of
a magnet intersect, then at the point of intersection, the compass needle
points in two different directions. This is not possible. Hence, two field
lines do not intersect each other.
The magnetic field in
a given region is uniform. Draw a diagram to represent it.
The magnetic field
lines inside a current-carrying long straight solenoid are uniform.
Consider a circular
loop of wire lying in the plane of the table. Let the current pass through the
loop clockwise. Apply the right-hand rule to find out the direction of the
magnetic field inside and outside the loop.
Inside the loop =
Pierce inside the table
Outside the loop =
Appear to emerge out from the table
For downward direction
of current flowing in the circular loop, the direction of magnetic field lines
will be as if they are emerging from the table outside the loop and merging in
the table inside the loop. Similarly, for upward direction of current flowing
in the circular loop, the direction of magnetic field lines will be as if they
are emerging from the table outside the loop and merging in the table inside
the loop, as shown in the given figure.
Choose the correct
option.
(d)The magnetic field inside a long, straight,
current-carrying solenoid is uniform. It is the same at all points inside the
solenoid.
When a proton enters
in a region of magnetic field, it experiences a magnetic force. As a result of
the force, the path of the proton becomes circular. Hence, its velocity and
momentum change.
State Fleming’s
left-hand rule.
Fleming’s left hand
rule states that if we arrange the thumb, the centre finger, and the forefinger
of the left hand at right angles to each other, then the thumb points towards
the direction of the magnetic force, the centre finger gives the direction of
current, and the forefinger points in the direction of magnetic field.
What is the principle
of an electric motor?
The working principle
of an electric motor is based on the magnetic effect of current. A
current-carrying loop experiences a force and rotates when placed in a magnetic
field. The direction of rotation of the loop is given by the Fleming’s
left-hand rule.
Question 1:
Explain different ways
to induce current in a coil.
The different ways to
induce current in a coil are as follows:
(a) If a coil is moved
rapidly between the two poles of a horse-shoe magnet, then an electric current
is induced in the coil.
(b) If a magnet is
moved relative to a coil, then an electric current is induced in the coil.
State the principle of
an electric generator.
An electric generator
works on the principle of electromagnetic induction. It generates electricity
by rotating a coil in a magnetic field.
Name some sources of
direct current.
Some sources of direct
current are cell, DC generator, etc.
Which sources produce
alternating current?
AC generators, power
plants, etc., produce alternating current.
Choose the correct
option.
Name two safety
measures commonly used in electric circuits and appliances.
Two safety measures
commonly used in electric circuits and appliances are as follows:
(i) Each circuit must
be connected with an electric fuse. This prevents the flow of excessive current
through the circuit. When the current passing through the wire exceeds the
maximum limit of the fuse element, the fuse melts to stop the flow of current
through that circuit, hence protecting the appliances connected to the circuit.
(ii) Earthing is a
must to prevent electric shocks. Any leakage of current in an electric
appliance is transferred to the ground and people using the appliance do not
get the shock.
Question 2:
An electric oven of 2
kW is operated in a domestic electric circuit (220 V) that has a current rating
of 5 A. What result do you expect? Explain.
Current drawn by the
electric oven can be obtained by the expression,
P = VI
Where,
Current = I
Power of the oven, P
= 2 kW = 2000 W
Voltage supplied, V
= 220 V
Hence, the current
drawn by the electric oven is 9.09 A, which exceeds the safe limit of the
circuit. Fuse element of the electric fuse will melt and break the circuit.
Question 3:
What precaution should
be taken to avoid the overloading of domestic electric circuits?
The precautions that
should be taken to avoid the overloading of domestic circuits are as follows:
(a) Too many
appliances should not be connected to a single socket.
(b) Too many
appliances should not be used at the same time.
(c) Faulty appliances
should not be connected in the circuit.
(d) Fuse should be
connected in the circuit.
Question 1:
Which of the following
correctly describes the magnetic field near a long straight wire?
(d) The magnetic field lines, produced around
a straight current-carrying conductor, are concentric circles. Their centres
lie on the wire.
Question 2:
The phenomenon of
electromagnetic induction is
(c) producing induced current in a coil due to
relative motion between a magnet and the coil
(c) When a straight coil and a magnet are
moved relative to each other, a current is induced in the coil. This phenomenon
is known as electromagnetic induction.
Question 3:
(a) An electric
generator produces electric current. It converts mechanical energy into
electricity.
(c) When two naked
wires of an electric circuit touch each other, the amount of current that is
flowing in the circuit increases abruptly. This causes short-circuit.
An electric motor
converts electrical energy into mechanical energy.
A generator is an
electric device that generates electricity by rotating a coil in a magnetic
field. It works on the principle of electromagnetic induction.
A long circular coil
is a long solenoid. The magnetic field lines inside the solenoid are parallel
lines.
Live wire has red
insulation cover, whereas earth wire has green insulation colour in the
domestic circuits.
Question 7:
List three sources of
magnetic fields.
Three sources of
magnetic fields are as follows:
(a) Current-carrying
conductors
(b) Permanent magnets
(c) Electromagnets
Question 8:
How does a solenoid
behave like a magnet? Can you determine the north and south poles of a
current-carrying solenoid with the help of a bar magnet? Explain.
A solenoid is a long
coil of circular loops of insulated copper wire. Magnetic field lines are
produced around the solenoid when a current is allowed to flow through it. The
magnetic field produced by it is similar to the magnetic field of a bar magnet.
The field lines produced in a current-carrying solenoid is shown in the
following figure.
In the above figure,
when the north pole of a bar magnet is brought near the end connected to the
negative terminal of the battery, the solenoid repels the bar magnet. Since
like poles repel each other, the end connected to the negative terminal of the
battery behaves as the north pole of the solenoid and the other end behaves as
a south pole. Hence, one end of the solenoid behaves as a north pole and the
other end behaves as a south pole.
Question 9:
When is the force
experienced by a current-carrying conductor placed in a magnetic field largest?
The force experienced
by a current-currying conductor is the maximum when the direction of current is
perpendicular to the direction of the magnetic field.
Question 10:
Imagine that you are
sitting in a chamber with your back to one wall. An electron beam, moving
horizontally from back wall towards the front wall, is deflected by a strong
magnetic field to your right side. What is the direction of magnetic field?
The direction of
magnetic field is given by Fleming’s left hand rule. Magnetic field inside the
chamber will be perpendicular to the direction of current (opposite to the
direction of electron) and direction of deflection/force i.e., either upward or
downward. The direction of current is from the front wall to the back wall
because negatively charged electrons are moving from back wall to the front
wall. The direction of magnetic force is rightward. Hence, using Fleming’s left
hand rule, it can be concluded that the direction of magnetic field inside the
chamber is downward.
Question 10:
Imagine that you are
sitting in a chamber with your back to one wall. An electron beam, moving
horizontally from back wall towards the front wall, is deflected by a strong
magnetic field to your right side. What is the direction of magnetic field?
The direction of
magnetic field is given by Fleming’s left hand rule. Magnetic field inside the
chamber will be perpendicular to the direction of current (opposite to the
direction of electron) and direction of deflection/force i.e., either upward or
downward. The direction of current is from the front wall to the back wall
because negatively charged electrons are moving from back wall to the front
wall. The direction of magnetic force is rightward. Hence, using Fleming’s left
hand rule, it can be concluded that the direction of magnetic field inside the
chamber is downward.
Question 13:
A coil of insulated
copper wire is connected to a galvanometer. What will happen if a bar magnet is
(i) pushed into the coil, (ii) withdrawn from inside the coil, (iii) held
stationary inside the coil?
A current induces in a
solenoid if a bar magnet is moved relative to it. This is the principle of
electromagnetic induction.
(i) When a bar magnet
is pushed into a coil of insulated copper wire, a current is induced
momentarily in the coil. As a result, the needle of the galvanometer deflects
momentarily in a particular direction.
(ii) When the bar
magnet is withdrawn from inside the coil of the insulated copper wire, a
current is again induced momentarily in the coil in the opposite direction. As
a result, the needle of the galvanometer deflects momentarily in the opposite
direction.
(iii) When a bar
magnet is held stationary inside the coil, no current will be induced in the
coil. Hence, galvanometer will show no deflection.
Question 14:
Two circular coils A
and B are placed closed to each other. If the current in the coil A is changed,
will some current be induced in the coil B? Give reason.
Two circular coils A
and B are placed close to each other. When the current in coil A
is changed, the magnetic field associated with it also changes. As a result,
the magnetic field around coil B also changes. This change in magnetic field
lines around coil B induces an electric current in it. This is called
electromagnetic induction.
Question 15:
State the rule to
determine the direction of a (i) magnetic field produced around a straight
conductor-carrying current, (ii) force experienced by a current-carrying
straight conductor placed in a magnetic field which is perpendicular to it, and
(iii) current induced in a coil due to its rotation in a magnetic field.
(i) Maxwell’s right
hand thumb rule
(ii) Fleming’s left
hand rule
(iii) Fleming’s right
hand rule
Question 16:
When does an electric
short circuit occur?
If the resistance of
an electric circuit becomes very low, then the current flowing through the
circuit becomes very high. This is caused by connecting too many appliances to a
single socket or connecting high power rating appliances to the light circuits.
This results in a short circuit.
When the insulation of
live and neutral wires undergoes wear and tear and then touches each other, the
current flowing in the circuit increases abruptly. Hence, a short circuit
occurs.
What is the function
of an earth wire? Why is it necessary to earth metallic appliances?
The metallic body of
electric appliances is connected to the earth by means of earth wire so that
any leakage of electric current is transferred to the ground. This prevents any
electric shock to the user. That is why earthing of the electrical appliances is
necessary.
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